Stainless steel for polymer fuel cell separator and method for preparing same

ABSTRACT

There are provided a ferrite stainless steel for a polymer fuel cell separator having excellent corrosion resistance and interfacial contact resistance under an operating environment of a polymer fuel cell, and a preparation method of the stainless steel. A stainless steel includes C: 0.02 wt % or less, N: 0.02 wt % or less, Si: 0.4 wt % or less, Mn: 0.2 wt % or less, P: 0.04 wt % or less, S: 0.02 wt % or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %, Ni: 0.8 wt % or less, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, waste Fe and inevitably contained elements. A preparation method of the stainless steel having a second passive film formed on a surface thereof includes forming a first passive film on the surface of the stainless steel by bright-annealing or annealing-pickling the stainless steel; removing the first passive film by pickling the stainless steel in a 10 to 20 wt % sulfuric acid solution at a temperature of 50 to 75° C. for a predetermined time; water-washing the stainless steel; and forming the second passive film by performing a passivation treatment on the stainless steel in the mixture of a 10 to 20 wt % nitric acid and a 1 to 10 wt % fluorine acid at a temperature of 40 to 60° C. for the predetermined time. Accordingly, it is possible to prepare a stainless steel having reduced elution resistance and excellent corrosion resistance and to produce a stainless steel for a polymer fuel cell separator, which has low interfacial contact resistance and excellent long-term performance even under a fuel cell operating condition of 60 to 150° C. and various surface roughness conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/142,495, filed Aug. 12, 2011 (now U.S. Pat. No. 9,290,845, issuedMar. 22, 2016), which is a 371 of international patent application no,PCT/KR2009/007891, filed Dec. 29, 2009, and claims the benefit of KoreanPatent Application No. 10- 2008-0135141, filed Dec. 29, 2008, and KoreanPatent Application No. 10-2009- 0127397, filed Dec. 18, 2009, thecontent of each application being incorporated herein by reference.

TECHNICAL FIELD

An aspect of the present invention relates to a stainless steel for afuel cell separator and method of manufacturing the same. Moreparticularly, an aspect of the present invention relates to a stainlesssteel for a fuel cell separator, which has low interfacial contactresistance and excellent corrosion resistance by setting surfacereforming conditions so as to remove an passive film of the stainlesssteel and control repassivation treatment even under various surfaceroughness conditions, and a manufacturing method of the stainless steel.

BACKGROUND ART

In recent years, the energy depletion, the environmental pollution andthe like have issued as global problems, and therefore, hydrogen energyand fuel cells using the hydrogen energy have been increasingly used asa substitute of fossil fuel. A fuel cell is a device that transformschemical energy of hydrogen into electrical energy. Since the fuel celldoes not use an internal-combustion engine, there is no noise andvibration, and high efficiency can be achieved. Since pollutants arehardly generated, the fuel cell has come into the spotlight as a newenergy source.

Fuel cells may be divided into a solid polymer fuel cell, a solid oxidefuel cell, a molten carbonate fuel cell, a phosphoric acid fuel cell, adirect methanol fuel cell and an alkaline fuel cell depending on thekind of an electrolyte. The fuel cells may be used as those for powergeneration, transportation, portable purposes, and the like.

In the solid polymer fuel cell (polymer electrolyte membrane fuel cell,or PEMFC), a solid polymer membrane is used as an electrolyte, and hencethe solid polymer fuel cell can operate at a normal temperature andpressure. Further, the solid polymer fuel cell has come into thespotlight as a power source because of a lower operating temperature ofabout 70 to 80° C., a short operating time and a high power density.Recently, a polymer fuel cell operable even at 100 to 150° C. has beenunder development.

FIG. 1 is a perspective view of a fuel cell including a stainless steelseparator.

Referring to FIG. 1, a solid polymer fuel cell stack 100 includes amembrane electrode assembly 110 including an electrolyte, electrodes (ananode and a cathode) and a gasket for gas sealing, a separator 120having a flow channel, and an end plate including inlets and outlets 130and 140 of air and inlets and outlets 150 and 160 of hydrogen gas.

The separator 120 is generally formed of any one of graphite, carbon, Tialloy, stainless steel and conductive plastic. Preferably, the separatoris formed of the stainless steel. The stainless steel has lowinterfacial contact resistance, excellent corrosion resistance, highheat conductivity, low gas transmittance. The stainless steel also hasexcellent mechanical strength and formability at a thin plate. Thus, thestainless steel has an advantage in that the volume and weight of thefuel cell stack can be reduced.

However, the separator 120 formed of the stainless steel has a problemin that the interfacial contact resistance between a material surface ofthe separator and a membrane electrode assembly (MEA) layer may beincreased by semiconducting characteristics of an passive film formed onthe surface of the separator 120 under fuel cell operating conditions.Further, the separator 120 requires excellent corrosion resistance inthe fuel cell operating environmental atmosphere having a strong acidicatmosphere.

In order to solve such a problem, U.S. Pat. No. 6,835,487B2 and KoreanPatent No. 0488922 disclose a method for obtaining surfacecharacteristics of a stainless steel to a desired level by regulatingthe mean surface roughness Ra that shows a surface roughness to be 001to 1.0 μm and regulating the maximum height Ry to be 0.01 to 20 μm so asto decrease the surface contact resistance of the stainless steel to 100mΩcm² or lower. Here, the stainless steel contains Cr (16 to 45 wt %)and Mo (0.1 to 3.0 wt %) and additionally contains Ag (0.001 to 0.1 wt%). Japanese Laid-Open Publication No 2007-026694 discloses a method forobtaining surface characteristics of a stainless steel to a desiredlevel by forming micro-pits of 0.01 to 1.0 μm on the entire surface ofthe stainless steel containing Cr and Mo. U.S. Pat. No. 6,379,476B1discloses a method for preparing a ferritic stainless steel having amean roughness of 0.06 to 5 μm by exposing carbide (carbide inclusion)and boride (boride inclusion) to a surface of the ferritic stainlesssteel. Here, the ferrite stainless steel contains 0.08% or more C forforming carbide. Japanese Laid-Open Publication No 2005-302713 disclosesa technique for ensuring the locally calculated mean interval S=0.3 μmor less and the second-order mean-square slope Δq=0.05 or more in astainless steel containing Cr (16 to 45 wt %) and Mo (0.1 to 5.0 wt %).

However, these methods are provided only for the purposes of decreasingcontact resistance by regulating the surface roughness of the stainlesssteel, micro-pits or conductive inclusions. To this end, the surfaceroughness of the stainless steel should be strictly maintained.Therefore, productivity is lowered, and production cost is increased.Further, it is difficult to secure reproducibility. In these methods, acomponent containing Cr and Mo as essential elements is specified withina predetermined range, and Ag, C and B for forming other conductiveinclusions are added as additional elements to the stainless steel.Therefore, the increase in preparation cost may be caused, and it is notnecessary to ensure stability of contact resistance and elutionresistance under the fuel cell operating condition (60 to 150° C.) inacidic environment.

Japanese Laid-Open Publication No 2004-149920 discloses a method forreducing contact resistance by regulating the Cr/Fe atomic ratio to be 1or more in a stainless steel containing Cr (16 to 45 wt %) and Mo (0.1to 5.0 wt %). Japanese Laid-Open Publication No 2008-091225 discloses amethod for reducing contact resistance by forming micro-pits in astainless steel containing Cr (16 to 45 wt %) and Mo (0.1 to 5.0 wt %)and by securing the Cr/Fe atomic ratio to be 4 or more.

However, these methods have difficulty in specifying a componentcontaining Cr and Mo as essential elements and stably securing lowinterfacial contact resistance without ensuring a strict control processof a passive film even under conditions having various surfaceroughnesses. Further, it is not necessary to ensure stability of contactresistance and elution resistance under the fuel cell operatingcondition (60 to 150° C.) in an acidic environment.

DISCLOSURE OF INVENTION Technical Problem

Accordingly, an object of the present invention is to provide astainless steel for a polymer fuel cell separator having excellentlong-term performance which can set surface reforming conditions so thatit is possible to maintain excellent elution resistance and contactresistance even under a fuel cell operating environment of 60 to 150° C.in an acidic environment and to secure low interfacial contactresistance and corrosion resistance by controlling the removal of apassive film and repassivation treatment even under various surfaceroughness conditions, and a preparation method of the stainless steel.

Technical Solution

According to an aspect of the present invention, there is provided astainless steel for a fuel cell separator, to which Mo is not added, thestainless steel comprising C: 0.02 wt % or less, N: 0.02 wt % or less,Si: 0.4 wt % or less, Mn: 0.2 wt % or less, P: 0.04 wt % or less, S:0.02 wt % or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %, Ni: 0.8 wt% or less, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, waste Fe andinevitably contained elements. In the stainless steel, a passive filmformed on a surface of the stainless steel is formed to have a thicknessof 2 to 4.5 nm, the Cr/Fe oxide ratio of the passive film is 1.5 or morein a region of 1.5 nm or less, and the Cr(OH)₃/Cr oxide distribution ofthe passive film has a ratio of 0 to 0.7 in a region of 1 nm.

The stainless steel may include Mo: 5.0 wt % or less.

The stainless steel may further include one or two or more elementsselected from the group consisting of V: 0.1 to 1.5 wt %, W: 0.1 to 2.0wt %, La: 0.0005 to 1.0 wt %, Zr: 0.0005 to 1.0 wt % and B: 0.0005 to1.0 wt %.

The contact resistance of the stainless steel may be 10 mΩcm² or less.

According to another aspect of the present invention, there is provideda preparation method of a stainless steel for a polymer fuel cellseparator, comprising C: 0.02 wt % or less, N: 0.02 wt % or less, Si:0.4 wt % or less, Mn: 0.2 wt % or less, P: 0.04 wt % or less, S: 0.02 wt% or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %, Ni: 0.8 wt % orless, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, waste Fe andinevitably contained elements, in which a second passive film is formedon a surface of the stainless steel to which Mo is not added, the methodcomprising: forming a first passive film on the surface of the stainlesssteel by bright-annealing or annealing-pickling the stainless steel;removing the first passive film by pickling the stainless steel in a 10to 20 wt % sulfuric acid solution at a temperature of 50 to 75° C. for atime controlled based on a surface roughness Ra; water-washing thestainless steel; and forming the second passive film by performing apassivation treatment on the stainless steel in the mixture of a 10 to20 wt % nitric acid and a 1 to 10 wt % fluorine acid at a temperature of40 to 60° C. for the time controlled based on the surface roughness Ra.

In the removing of the first passive film, the stainless steel may bepickled for a processing time according to the following expression:99−3.18(1/Ra)≤processing time (t, second)s≤153−3.18(1/Ra).

In the forming of the second passive film, the stainless steel may besubjected to the passivation treatment for a processing time accordingto the following expression:120+6.73(1/Ra)≤processing time(t, second)≤140+6.73(1/Ra).

The contact resistance of the stainless steel may be 10 mΩcm² or lessunder an operating environment of 60 to 150° C.

In the forming of the second passive film, the second passive film maybe formed to have a thickness of 2 to 4.5 nm.

In the forming of the second passive film, the Cr/Fe oxide ratio of thesecond passive film may be 1.5 or more in a region of 1.5 nm or less.

In the forming of the second passive film, the Cr(OH)₃/Cr oxidedistribution of the second passive film may have a ratio of 0 to 0.7 ina region of 1 nm.

Advantageous Effects

As described above, according to the present invention, surfacereforming conditions are set so that it is possible to control theremoval of a passive film of a stainless steel and repassivationtreatment even under various surface roughness conditions. Thus, thestainless steel can secure low interfacial contact resistance andachieve excellent corrosion resistance with reduced elution resistance.Accordingly, it is possible to produce a stainless steel havingexcellent long-term performance of the polymer fuel cell.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a fuel cell including a generalstainless separator.

FIG. 2 is a graph showing a change in initial interfacial contactresistance for each surface roughness.

FIG. 3 is a graph showing a change in potential obtained by immersinginvention steel 12 in a 15 wt % sulfuric acid solution at 70° C. using asaturated calomel electrode (SCE) electrode as a reference electrode.

FIG. 4 is a graph showing a change in potential when washing thestainless steel that goes through FIG. 3 and then immersing theinvention steel 12 in the mixture of a nitric acid of 15 wt % and afluorine acid of 5 wt % using the SCE electrode as a referenceelectrode.

FIG. 5 is a graph showing a change in contact resistance whenheat-treating a stainless steel subjected to surface reforming treatmentunder an air atmosphere.

FIGS. 6A to 6C are graphs showing examples of performing an X-rayphotoelectron microscopy analysis on composition distributions of firstand second passive films in an initial period, after performing asurface reforming treatment and after performing a potentiostaticpolarization test according to embodiments of Table 2.

FIGS. 7A to 7C are graphs showing Cr/Fe oxide distributions in the firstand second passive films in the initial period, after performing thesurface reforming treatment and after performing the potentiostaticpolarization test according to the embodiments of Table 2.

FIG. 8 is a graph showing a Cr(OH)₃/Cr oxide distribution in the firstand second passive films in the initial period, after performing thesurface reforming treatment and after performing the potentiostaticpolarization test according to the embodiments of Table 2.

FIGS. 9A to 9C are graphs showing performance estimation resultsmeasured by preparing stainless separators formed by performing anoptimum two-step surface reforming treatment on invention steels 9 and12 in Table 3 and then assembling the prepared stainless separator in apolymer fuel cell unit cell.

BEST MODE

Hereinafter, a stainless steel for a fuel cell separator according tothe present invention will be described in detail with reference to theaccompanying drawings.

A general stainless steel for a fuel cell separator having varioussurface roughness conditions, such as a bright annealing material orannealing pickling material of a stainless steel cold-rolled material,has high contact resistance due to a passive film formed on a surface ofthe stainless steel after annealing and pickling the stainless steel,and the surface roughness conditions are partially changed due tofriction between the surface and a mold in the molding process of aportion of the separator. Accordingly, an appropriate surface reformingtreatment is preferably performed on the stainless steel for theseparator so as to satisfy requirements of a separator having lowcontact resistance and improved corrosion resistance under varioussurface roughness conditions.

To this end, in the present invention, a stainless steel for a polymerfuel cell separator, which has low interfacial contact resistance andexcellent corrosion resistance, and a preparation method of thestainless steel, will be described. A separator is stamed orhydro-formed using a stainless steel having various surface roughnesses.In order to remove a first passive film formed on the separator, thestainless steel is pickled in a 10 to 20 wt % sulfuric acid solution ata temperature of 50 to 75° C. under an optimum processing conditionwhile maintaining an appropriate time depending on a surface roughnesscondition. After water-washing the stainless steel, a repassivationtreatment of the stainless steel is performed in the mixture of a 10 to20 wt % nitric acid and a 1 to 10 wt % fluoric acid at a temperature of40 to 601° C. while maintaining an appropriate time depending on asurface roughness condition so as to form a second passive film.Accordingly, the thickness of the passive film formed on the surface ofthe stainless steel is formed to 2 to 4.5 nm, and the Cr/Fe oxide ratiois 1.5 or more in a region of less than 1.5 nm. Also, the Cr(OH)₃/Croxide ratio is 0 to 0.7 in a region of 1 nm, so that the contactresistance of the stainless steel can ensure 10 mΩcm² or less.

Hereinafter, the stainless steel for the polymer fuel cell separator,which has low interfacial contact resistance and excellent corrosionresistance, will be described in detail.

The stainless steel according to the present invention has a compositionof elements including C: 0.02 wt % or less, N: 0.02 wt % or less, Si:0.4 wt % or less, Mn: 0.2 wt % or less, P: 0.04 wt % or less, Cr: 25.0to 32.0 wt %, Cu: 0 to 0.2 wt %, Ni: 0.8 wt % or less, and Ti: 0.5 wt %or less; one or two or more elements selected from the group consistingof V: 0 to 1.5 wt %, W: 0 to 2.0 wt %, La: 0 to 1.0 wt %, Zr: 0 to 1.0wt % and B: 0 to 0.1 wt %; and waste Fe and inevitably containedelements. As described above, Mo is not added to the stainless steelaccording to the present invention.

Meanwhile, in a case where the Mo is added to the stainless steel, thecontent of the Mo is preferably 0.5 wt % or less.

A slab produced by steel making, refining and continuous casting analloy with such a composition passes through processes of hot-rolling,annealing, pickling, cool-rolling, annealing and pickling, therebypreparing a cold-rolled product.

Hereinafter, the composition range and limitation reason of the presentinvention will be described in detail. The following percentages (%) areweight percentages in all cases.

C and N form a Cr carbon/nitride in the stainless steel. As a result,the corrosion resistance of a layer in which Cr is deficient is lowered,and therefore, both the elements are preferably contained as small aspossible. Thus, in the present invention, their composition ratios arelimited to C: 0.02% or less, and N: 0.02% or less, respectively.

Si is an element effective for deoxidation but constraints ductility andformability. Thus, in the present invention, the composition of the Siis limited to 0.4% or less.

Mn is an element that increases deoxidation, but MnS that is aninclusion decreases corrosion resistance. Thus, in the presentinvention, the composition ratio of the Mn is limited to 0.2% or less.

P decreases ductility in addition to corrosion resistance. Thus, in thepresent invention, the composition ratio of the P is limited to 0.04% orless.

S forms MnS, and the MnS becomes a starting point to decrease corrosionresistance. Thus, in the present invention, the composition ratio of theS is limited to 0.02% or less.

Cr increases corrosion resistance under an acidic atmosphere in which afuel cell is operated. But Cr decreases ductility. Thus, in the presentinvention, the composition ratio of the Cr is limited to 25 to 32%.

Mo functions to increase corrosion resistance under the acidicatmosphere in which the fuel cell is operated. However, if the Mo isexcessively added to the stainless steel, ductility is decreased, andthe Mo is disadvantageous in terms of effect and economic efficiency.Thus, in the present invention, the Mo is not basically added to thestainless steel. The present invention can obtain a desired effect evenwhen the Mo is not added to the stainless steel. In a case where it isparticularly required to improve corrosion resistance, the Mo may beseparately added to the stainless steel. In this case, the content ofthe Mo is preferably limited to the range of 5% or less.

Cu may increase corrosion resistance under the acidic atmosphere inwhich the fuel cell is operated, or the performance of the fuel cell andthe formability of the stainless steel may be lowered due to the elutionof the Cu and the effect of solid solution strength when the Cu isexcessively added to the stainless steel. Thus, in the presentinvention, the composition ratio of the Cu is limited to the range from0 to 2%.

Ni functions to decrease partial contact resistance, but the election ofthe Ni and the formability of the stainless steel may be lowered whenthe Ni is excessively added to the stainless steel. Thus, in the presentinvention, the composition ratio of the Ni is limited to 0.8% or less.

Ti and Nb are element effective for forming C and N as a carbon nitridein the stainless steel, but decrease ductility. Thus, in the presentinvention, the composition ratio of each of the Ti and Nb is limited to0.5% or less.

In addition, one or two or more of V, W, La, Zr and B may be added tothe stainless steel, and their composition ratios are as follows.

V increases corrosion resistance under the acidic atmosphere in whichthe fuel cell is operated, but the performance of the fuel cell may belowered due to the elution of ions when the V is excessively added tothe stainless steel. Thus, in the present invention, the compositionratio of the V is limited to 0.1 to 1.5%.

W increases corrosion resistance and decreases interfacial contactresistance under the acidic atmosphere in which the fuel cell isoperated. However, when the W is excessively added to the stainlesssteel, tension is decreased. Thus, in the present invention, thecomposition ratio of the W is limited to 0.1 to 2.0%.

La may induce fine dispersion of a sulfide-based inclusion in thestainless steel and induce densification of a passive film. However,when the La is excessively added to the stainless steel, there occurs aproblem of nozzle clogging, or the like. Thus, in the present invention,the composition ratio of the La is limited to 0.0005 to 1.0%.

Zr increases corrosion resistance under the acidic atmosphere in whichthe fuel cell is operated, but induces a surface defect when the Zr isexcessively added to the stainless steel. Thus, in the presentinvention, the composition ratio of the Zr is limited to 0.0005 to 1.0%.

B forms a nitride in the stainless steel and improves its corrosionresistance. However, when the B is excessively added to the stainlesssteel, a surface defect is induced. Thus, in the present invention, thecomposition ratio of the B is limited to 0.0005 to 1.0%.

Hereinafter, a process of setting a processing condition for eachsurface roughness condition.

The compositions of stainless steels prepared in the present inventionare shown in Table 1.

TABLE 1 Kind of steel C Si Mn P S Al Cr Ni Cu Ti Nb Mo Other N Invention0.009 0.29 0.142 <0.003 <0.002 0.048 26.26 0.173 0.437 0.058 0.241 2.03— 0.009 steel 1 Invention 0.007 0.273 0.146 <0.003 <0.002 0.058 26.330.167 0.436 0.059 0.25 4.04 — 0.009 steel 2 Invention 0.008 0.269 0.145<0.003 <0.002 0.055 30.01 0.169 0.43 0.06 0.245 2.13 — 0.009 steel 3Invention 0.006 0.293 0.143 <0.003 <0.002 0.041 30.29 0.168 0.4 0.0460.242 4.03 — 0.008 steel 4 Invention 0.006 0.303 0.14 <0.003 <0.0020.027 28.55 0.181 0.412 0.041 0.241 3.19 — 0.009 steel 5 Invention 0.0080.341 0.146 <0.003 <0.002 0.039 30.66 0.181 0.442 0.041 0.245 2.030.0013La 0.012 steel 6 Invention 0.008 0.307 0.14 <0.003 <0.002 0.0329.68 0.19 0.421 0.039 0.24 2.12 0.420W 0.008 steel 7 Invention 0.0080.263 0.145 <0.003 <0.002 0.055 29.68 0.184 0.429 0.051 0.243 2 0.002Zr0.008 steel 8 Invention 0.007 0.24 0.151 <0.003 <0.002 0.041 30.42 0.1850.964 0.05 0.247 2.03 — 0.009 steel 9 Invention 0.009 0.267 0.148 <0.003<0.002 0.04 30.75 0.183 0.455 0.049 0.243 2.03 0.363V 0.009 steel 10Invention 0.007 0.308 0.15 <0.003 <0.002 0.021 30.42 0.183 0.442 0.0360.24 2.01 0.001B 0.008 steel 11 Invention 0.003 0.107 0.146 0.003 <0.0030.057 30.05 0.192 0.977 0.05 0.25 2 0.4W 0.006 steel 12 Invention 0.0040.199 0.153 0.003 <0.003 0.043 30.1 0.193 1.01 0.045 0.25 1.98 0.41W0.007 steel 13 Invention 0.005 0.222 0.148 0.003 <0.003 0.056 30.260.207 0.97 0.048 0.26 2.03 0.4V 0.009 steel 14 Invention 0.005 0.1620.152 0.003 <0.003 0.946 29.96 0.747 0.975 0.052 0.26 2 0 0.005 steel 15Invention 0.006 0.208 0.138 0.004 <0.003 0.03 30.07 0.204 0.973 0.0410.25 1.47 0.41W 0.007 steel 16 Invention 0.004 0.2 0.145 0.003 <0.0030.047 28.01 0.199 0.993 0.044 0.25 2.01 0.41W 0.008 steel 17 Invention0.006 0.141 0.152 <0.003 <0.003 0.051 30.25 0.11 1.00 0.049 0.26 — 0.39V0.011 steel 18 Invention 0.011 0.138 0.167 <0.003 <0.003 0.071 30.580.11 0.96 0.056 0.27 — 0.40V 0.012 steel 19 Invention 0.006 0.118 0.151<0.003 <0.003 0.049 30.20 0.11 0.97 0.051 0.26 — — 0.008 steel 20Invention 0.005 0.136 0.150 <0.003 <0.003 0.034 30.16 0.12 0.96 0.0510.25 0.98 0.40W 0.009 steel 21 Invention 0.006 0.126 0.150 <0.003 <0.0030.043 30.16 0.10 0.97 0.050 0.25 — — 0.009 steel 22 Invention 0.0080.110 0.151 <0.003 <0.003 0.083 30.19 0.11 0.98 0.056 0.26 — 0.37W 0.009steel 23 Invention 0.004 0.124 0.121 <0.003 <0.003 0.037 30.10 0.12 —0.051 0.24 — 0.4V 0.008 steel 24 Invention 0.005 0.113 0.143 <0.003<0.003 0.035 30.11 0.11 — 0.050 0.25 — — 0.009 steel 25 Invention 0.0070.128 0.128 <0.003 <0.003 0.941 30.01 — 0.97 0.050 0.25 — 0.4V 0.009steel 26 Invention 0.006 0.111 0.137 <0.003 <0.003 0.025 30.00 — 0.980.056 0.26 — — 0.09 steel 27 Comparative 0.008 0.4 0.34 0.003< <0.0020.003 19.33 0.14 0.45 — 0.43 0.01 0.98V 0.008 steel

The inventor measured an initial interfacial contact resistance at acontact pressure of 140 N/cm² with respect to each of the steels inTable 1, and measured its interfacial contact resistance aftercompleting chemical surface reforming according to the presentinvention. The measurement of the interfacial contact resistance will beillustrated in the following description.

In order to observe a change in contact resistance depending on asurface roughness, the inventor prepared stainless steel plates havingdifferent surface roughnesses with respect to the invention steel 12 inTable 1 as a representative example and then measured the interfacialcontact resistance in the state of a passive film formed in the air.

The measurement of the interfacial contact resistance is measured by theDC 4-terminal method. Two separators are sandwiched between carbonpapers (SGL Co., GDL 10-BA), and a copper end plate is mounted togetherwith the carbon papers. Then, a current applying terminal is connectedto the copper end plate, and voltage terminals are connected to therespective two separators, thereby measuring a contact resistanceaccording to pressure. In this instance, the measurement of theinterfacial contact resistance for each of the slabs was performed fourtimes or more.

FIG. 2 is a graph showing a change in initial interfacial contactresistance for each surface roughness.

Referring to FIG. 2, the surface contact resistance of a stainless steelis decreased as the mean roughness Ra of a surface of the stainlesssteel, measured by surface roughness tester, is increased. Althoughstainless steels have the same kind of steel, they show a considerabledifference according to their surface roughnesses. The change ininterfacial contact resistance depending on surface roughness has acorrelation in that the contact resistance is in proportion to 1/Ra,1/Rq, 1/Rp, 1/Rt or 1/dp. However, the contact resistances of stainlesssteels of which mean roughnesses Ra are 0.350, 0.055 and 0.040,respectively, are high in applying the stainless steels as polymer fuelcell separators. When the contact resistance of a stainless steel isgenerally 10 mΩcm² or less, the stainless steel is suitable for theseparator.

That is, such a result means that it is difficult to secure a contactresistance of 10 mΩcm² or less only through the control of the surfaceroughness. As the result studied by the inventor, it has been found thatthis is result from a thin protective passive film formed on the surfaceof the stainless steel. The passive film is formed of a Fe—Cr-basedoxide and has a high content of Fe. Since the thickness of the passivefilm is thick, the contact resistance of the stainless steel is high.The stainless steel having a passive film formed thereon is not suitableto be used as a separator for a polymer fuel cell. Accordingly, theinvention has found that the passive film is necessarily removed fromthe stainless steel. Particularly, it is required to develop a techniquefor controlling the composition and thickness of a passive filmregardless of an initial surface roughness condition.

Thus, in the present invention, a stainless steel having a first passivefilm formed thereon was pickled in a 10 to 20 wt % sulfuric acidsolution at a temperature of 50 to 75° C. under optimum processingconditions while maintaining the following processing time according tosurface roughness conditions, thereby removing the first passive film.

FIG. 3 is an embodiment of an optimum pickling condition for eachsurface roughness according to the present invention. FIG. 3 is a graphshowing a change in potential obtained by immersing invention steel 12in a 15 wt % sulfuric acid solution at 70° C. using a saturated calomelelectrode (SCE) electrode as a reference electrode.

As shown in FIG. 3, the potential of the stainless steel in a state thatthe first passive film is formed on a surface of the invention steel 12is higher than that in a state that an oxide is not formed on thesurface of the invention steel 12. If the first passive film is removedby immersing the first passive film in the sulfuric acid solution, thepotential of the stainless steel is rapidly decreased within 25 seconds.This means that the removal of the first passive film composed of anoxide formed on the surface of the stainless steel is started, and as aresult, the potential of the stainless steel is gradually decreasedwithin 25 seconds. After a certain period of time elapses, the oxideformed on the surface of the immersed stainless steel is removed, sothat the potential of the stainless steel is not decreased any more butsaturated. Thus, the stainless steel is immersed in the sulfuric acidsolution from the potential lower than that in the initial period ofimmersion to the saturation time, thereby removing the first passivefilm (oxide) formed on the surface of the stainless steel.

Through the graph of FIG. 3, it can be seen that the aspect in whichstainless steel plate reacts in the sulfuric acid solution is differentfor each surface roughness condition, and accordingly, the processingcondition for removing an oxide in the sulfuric acid solution is alsodifferent for each of the surface roughness conditions. Therefore, inthe invention steel, the appropriate processing time for removing thefirst passive film in a 10 to 20 wt % sulfuric acid solution at atemperature of 50 to 75° C. is increased as the surface roughness isincreased. The appropriate processing time satisfies the followingexpression 1.99−3.18(1/Ra)≤processing time(t, second)≤153−3.18(1/Ra)  (1)

In a case where the temperature and concentration of the sulfuric acidsolution is extremely low, the oxide formed on the surface is not easilyremoved. On the contrary, in a case where temperature and concentrationof the sulfuric acid solution is extremely high, damage of a basematerial may be caused. Thus, the temperature of the sulfuric acidsolution is limited to 50 to 750° C., and the concentration of thesulfuric acid solution is limited to 10 to 20 wt %. If the processingtime is over the aforementioned conditions, it is difficult to removethe passive film with high interfacial contact resistance. If theprocessing time is below the aforementioned conditions, it is difficultto secure a contact resistance of 10 mΩcm² or less due to the damage ofthe base material and the formation of the passive film with highinterfacial contact resistance.

Then, the stainless steel having the first passive film removedtherefrom is water-washed.

A repassivation treating process is performed in a mixture of a 10 to 20wt % nitric acid and a 1 to 10 wt % fluoric acid at a temperature of 40to 60° C. while maintaining the processing time according to the surfaceroughness condition, thereby forming a second passive film on thesurface of the stainless steel.

FIG. 4 is an embodiment in which an optimum second passive film isformed for each surface roughness. FIG. 4 is a graph showing a change inpotential when washing the stainless steel that goes through FIG. 3 andthen immersing the invention steel 12 in a mixture of a nitric acid of15 wt % and a fluorine acid of 5 wt % using the SCE electrode as areference electrode.

Referring to FIG. 4, slabs with different surface roughness conditionsare water-washed by applying the optimum immersion condition of thesulfuric acid solution, derived from FIG. 3, and the second passive filmis formed on the surface of the stainless steel in the mixture of thenitric acid of 15 wt % and the fluorine acid of 5 wt %.

In a case where the stainless steel is immersed in an oxidizing acidsuch as the mixture of the nitric acid and the fluoric acid, a passivefilm is formed on the surface of the stainless steel. If the passivefilm is formed on the surface of the stainless steel, the potential ofthe stainless steel is increased as time elapses. As described above, ifthe stainless steel according to the present invention is immersed inthe mixture of the nitric acid and the fluoric acid from the potentialhigher than that in the initial period of immersion to the saturation,the second passive film is formed on the surface of the stainless steel.

In this process, the inventor has found that it takes much time toperform passivation treatment as the temperature of the passivationtreatment is decreased, and it is harmful to contact resistance andcorrosion resistance due to surface damage as the temperature of thepassivation treatment is increased. Thus, in the second passivationtreatment, the second passive film is preferably formed in a mixture ofa 10 to 20 wt % nitric acid and a 1 to 10 wt % fluoric acid at atemperature of 40 to 60° C. The inventor has also found that the secondpassive film is necessarily formed differently depending on the surfaceroughness condition, and it is possible to prepare a separator havingcharacteristics in which the processing time is decreased as the surfaceroughness is increased and the contact resistance is low at thefollowing processing time t. The preferred processing time satisfies thefollowing expression 2.120−6.73(1/Ra)≤processing time(t, second)≤140−6.73(1/Ra)  (2)

In the present invention, the concentration of the nitric acid islimited to 10 to 20 wt %. In a case where the concentration of thenitric acid is less than 10 wt %, it is difficult to perform the passivetreatment. In a case where the concentration of the nitric acid isextremely high, there is no effect of reduction in contact resistance.

In the present invention, the concentration of the fluoric acid islimited to 1 to 10 wt %. In a case where the concentration of thefluoric acid is less than 1 wt %, the passive film may be unstable. Onthe contrary, in a case where the fluoric acid is excessively added tothe stainless steel, it is harmful to contact resistance and corrosionresistance due to surface damage. In the present invention, the inventorhas found that when the processing time is deviated from an appropriatetime for each of the surface roughnesses, the passive film is unstableor the thickness of the passive film is excessively increased, andtherefore, the contact resistance is increased.

Accordingly, the stainless steel according to the present invention cansecure the initial contact resistance of 10 mΩcm² or less and thecontact resistance after performing a corrosion test under environmentalconditions of the fuel cell. Particularly, in the corrosion resistancewhen chemical surface reforming treatment is performed on theseinvention steels, the corrosion current density of the stainless steelis low, and the elution resistance of the stainless steel is excellent.The corrosion resistance will be described later in Table 3.

In the process of preparing a real fuel cell separator, the separatorcan be exposed up to 250 degrees in the bonding process of a sealingportion, and the temperature of the separator is increased by heatgenerated in separator welding such as laser welding. As ahigh-temperature electrolyte has recently been developed, the operatingtemperature is increased up to 150° C. Thus, in a case where the surfacereforming treatment is performed on the invention steel under thedevelopment condition, the stability of contact resistance is observed.As a result, all the invention steels can secure the contact resistanceof 10 mΩcm² or less. An example of the invention steel 18 is shown inFIG. 5.

As an embodiment of the present invention, table 2 shows a change ininterfacial contact resistance according to each processing time foreach surface roughness condition of the invention steel 12.

TABLE 2 Processing time Contact Processing time (second) of resistance(second) of 15 wt % nitric (mΩcm²) at 15 wt % sulfuric acid + 5 wt %pressure of Ra acid fluoric acid 140 N/m² Remark 0.350 0 0 28.26Comparative example 100 150 3.89 Invention example 0.055 0 0 75.37Comparative example 0 250 22.51 Comparative example 30 250 15.01Comparative example 60 250 3.94 Invention example 90 250 8.72 Inventionexample 0.040 0 0 102.44 Comparative example 30 300 3.89 Inventionexample

As shown in Table 2, when the mean surface roughness Ra is 0.055, thecontact resistance is changed by constantly maintaining the processingtime of the mixture of the nitric acid and the fluorine acid andmodifying the process of sulfuric acid pickling. Accordingly, theinventor has found that the process of sulfuric acid pickling has greatinfluence on the contact resistance. As the processing time is long orshort, the contact resistance is increased. Thus, the inventor has foundthat the processing time for each surface roughness condition hasimportant influence on the contact resistance.

For each of the steels in Table 1, the inventor performed a picklingprocess of a first passive film in a 10 to 20 wt % sulfuric acidsolution at a temperature of 50 to 75° C. for an appropriate processingtime [99−3.18(1/Ra)≤processing time (t, second)≤153−3.18(1/Ra)] and thenperformed a water-washing process. Then, the inventor performed aforming process of a second passive film in the mixture of a 10 to 20 wt% nitric acid and a 1 to 10 wt % fluoric acid at a temperature of 40 to60° C. for an appropriate time [120−6.73(1/Ra)≤processing time (t,second)≤140−6.73(1/Ra)]. Table 3 shows results for a contact resistancemeasured in the aforementioned process, a corrosion current densityafter performing a condition similar to a cathode atmosphere conditionin the atmosphere of the polymer fuel cell, i.e., a potentiostaticpolarization test, a contact resistance after performing thepotentiostatic polarization test, and a corrosion-resistance experimentwith respect to each of the invention steels. Here, the potentiostaticpolarization test is performed by applying a voltage of 0.6V for 9 hoursusing an SCE as a reference electrode while bubbling air in a mixedsolution of a sulfuric acid of IM and a fluorine acid of 2 ppm at atemperature of 70° C. The corrosion-resistance experiment is performedby measuring Fe, Cr and Ni elution ions in a corrosion solution using aninductively coupled plasma spectroscopy (ICP).

TABLE 3 Contact Contact resistance (mΩcm²) Elution ion resistance(mΩcm²) at 140 N/cm² after Corrosion Interfacial contact concentration(mg/L) at 140 N/cm² after heat treatment in current resistance (mΩcm²)after polarization Kind of chemical surface air atmosphere of 200density at 140 N/cm² after experiment steel reforming degrees for 20minutes (μA/cm²) polarization experiment Fe Cr Ni Invention 5.17 5.400.08 5.84 0.023 No No steel 1 Invention 4.32 5.1 0.02 4.92 0.033 No Nosteel 2 Invention 4.09 4.30 0.07 4.71 0.024 No No steel 3 Invention 4.354.55 0.03 4.52 0.017 No No steel 4 Invention 4.16 4.20 0.02 4.10 0.021No No steel 5 Invention 4.16 4.30 0.02 5.66 0.025 No No steel 6Invention 3.70 4.55 0.04 4.99 0.018 No No steel 7 Invention 3.77 4.540.09 4.78 0.017 No No steel 8 Invention 3.89 4.12 0.08 4.28 0.016 No Nosteel 9 Invention 3.74 4.31 0.03 4.23 0.019 No No steel 10 Invention3.87 4.55 0.03 4.38 0.027 No No steel 11 Invention 5.84 5.90 0.10 5.560.021 No No steel 12 Invention 4.06 5.10 0.12 8.52 0.024 No No steel 13Invention 4.25 4.95 0.09 5.68 0.040 No No steel 14 Invention 4.45 5.120.12 5.58 0.045 No No steel 15 Invention 5.33 5.78 0.12 6.07 0.049 No Nosteel 16 Invention 5.64 5.99 0.12 6.87 0.031 No No steel 17 Invention4.67 4.78 0.04 5.56 0.021 No No steel 18 Invention 5.82 5.97 0.08 6.520.022 No No steel 19 Invention 4.25 4.32 0.09 6.05 0.020 No No steel 20Invention 4.50 5.12 0.03 7.10 0.017 No No steel 21 Invention 5.12 5.780.06 5.45 0.023 No No steel 22 Invention 5.75 5.90 0.04 6.10 0.027 No Nosteel 23 Invention 4.35 4.44 0.03 5.78 0.017 No No steel 24 Invention4.67 4.78 0.02 5.99 0.020 No No steel 25 Invention 4.89 5.12 0.05 6.000.019 No No steel 26 Invention 5.01 5.09 0.03 5.79 0.021 No No steel 27Comparative 8.5 25.78 0.16 5 2.135 0.505 0.005 steel

As shown in Table 3, the contact resistance of the stainless steel forthe separator according to the embodiment of the present invention,obtained by exposing an invention steel subjected to chemical surfacereforming treatment at a temperature of 200° C. for 20 minutes under anair atmosphere, is measured at a contact pressure of 140 N/cm². As aresult, the stainless steel has a low contact resistance of 10 mΩcm² ascompared with the comparative steel. The contact resistance measuredafter performing the polarization experiment is lower than that of thecomparative steel. The corrosion current density measured afterperforming the polarization experiment also has a low value of 0.12μA/cm². As the result obtained by measuring elution ions, only Feelution ions of 0.05 mg/L or less are detected in the stainless steel ascompared with the comparative steel. Particularly, although Mo was usedas an essential element in the conventional patent, the invention steelto which the Mo is not added shows excellent contact resistance andelution resistance in the present invention. As an example, it can beseen that the invention steels 18, 19, 20, 22 and 23 have excellentcontact resistance and corrosion resistance in the present invention.

According to the embodiment of the present invention, an X-rayphotoelectron microscopy (XPS) analysis is performed on the secondpassive film obtained by performing two-step chemical surface reformingunder the aforementioned surface roughness condition and then performingthe potentiostatic polarization test.

FIGS. 6A to 6C are graphs showing examples of performing an X-rayphotoelectron microscopy analysis on composition distributions of firstand second passive films in an initial period, after performing asurface reforming treatment and after performing a potentiostaticpolarization test according to embodiments of Table 2.

Referring to FIGS. 6A to 6C, the thickness of the initial first passivefilm is about 5.5 nm, and the thickness of the second passive filmproduced after performing the two-step chemical surface reforming isabout 2.2 nm. It can be seen that the thickness of the second passivefilm after performing the potentiostatic polarization test is about 2.3nm, which is hardly different from the thickness of the second passivefilm produced after performing the two-step chemical surface reforming.Particularly, the thickness of the second passive film after performingthe chemical surface reforming on the invention steel in Table 3 wasmeasured within the range from 2 to 3.5 nm.

FIGS. 7A to 7C are graphs showing Cr/Fe oxide distributions in the firstand second passive films in the initial period, after performing thesurface reforming treatment and after performing the potentiostaticpolarization test according to the embodiments of Table 2.

Referring to FIGS. 7A to 7C, it can be seen that in addition to adecrease in thickness, the Cr/Fe oxide ratio in the second passive filmafter performing a surface reforming treatment and then performing apotentiostatic polarization test secures 1.5 or more in a region of 1nm, as compared with the Cr/Fe oxide ratio in the initial first passivefilm. Particularly, it can be seen that the Cr/Fe oxide ratio afterperforming the chemical surface reforming on the invention steel inTable 3 secures 1.5 or more in a region of less than 1.5 nm.

FIG. 8 is a graph showing a Cr(OH)₃/Cr oxide distribution in the firstand second passive films in the initial period, after performing thesurface reforming treatment and after performing the potentiostaticpolarization test according to the embodiments of Table 2.

As shown in FIG. 8, it can be seen that the Cr(OH)₃/Cr oxidedistribution in the second passive film after performing the two-stepsurface reforming and then performing the potentiostatic polarizationtest has a ratio of 0 to 0.52 in a region of less than 1 nm as comparedwith the Cr(OH)₃/Cr oxide ratio of the initial first air-formed filmwithin a region in which the thickness of the passive film is less than1 nm. Particularly, it can be seen that the Cr(OH)₃/Cr oxidedistribution after performing the two-step chemical surface reforming onthe invention steel in Table 3 has a ratio of 0 to 0.7 in a region ofless than 1 nm.

From the aforementioned experiment, it can be seen that the initialsurface roughness acts as a primary factor for setting conditions suchas processing time in the two-step surface reforming including apickling process of removing the first passive film in the sulfuric acidsolution of the stainless steel with the composition range of thepresent invention and a passivation treatment process of forming thesecond passive film in the mixed solution of the nitric acid and thefluorine acid after water-washing. The thickness of the second passivefilm of the stainless steel processed by the two-step chemical surfacereforming is formed in the range from 2 to 4.5 nm. Further, it can beseen that the Cr/Fe oxide ratio is 1.5 or more in a region of less than1.5 nm, and it is possible to ensure a stainless steel having excellentcontact resistance and corrosion resistance when the Cr(OH)₃/Cr oxidedistribution has a ratio of 0 to 0.7 in a region of less than 1 nm.

FIGS. 9A to 9C are graphs showing performance estimation resultsmeasured by preparing stainless separators formed by performing anoptimum two-step surface reforming treatment on invention steels 9 and12 in Table 3 and then assembling the prepared stainless separator in apolymer fuel cell unit cell.

Referring to FIGS. 9A to 9C, the operating temperature of the fuel cellwas maintained as 70, and the entire pressure of reactive gas wasmaintained as 1 atmospheric pressure. The amounts of hydrogen and oxygensupplied to an anode and a cathode were measured by supplying 1.5 of theamount of electrochemically consumed hydrogen and 2 of the amount ofelectrochemically consumed oxygen, respectively. A membrane electrodeassembly (produced by Gore Co.) was used as the used membrane electrodeassembly was used. In the long-term measurement, the voltage of a unitcell was measured under a standard condition while maintaining thecurrent density to 0.7 mA/cm² (17.5 A).

As shown in FIG. 9A, the initial performance shows a performance resultalmost similar to that of a graphite material. As shown in FIG. 9B, theohmic resistance and the polarization resistance are almost similar toor partially lower than those of the graphite material in the impedanceanalysis result. Referring to FIG. 9C, in terms of the long-termperformance measured for 600 hours, the open circuit voltage (OCV) ismaintained constant, and the voltage drop at a constant current densityof 0.7 mA/cm² (17.5 A) is identical to or partially higher than that ofthe graphite material.

That is, according to the embodiments of the invention steel, surfacereforming conditions are set so that it is possible to control theremoval of the passive film of the stainless steel and the repassivationtreatment of the stainless steel. Thus, the stainless steel can securelow interfacial contact resistance and achieve excellent corrosionresistance with reduced elution resistance. Accordingly, it is possibleto produce a stainless steel having excellent long-term performance ofthe polymer fuel cell.

In the aforementioned embodiment, the polymer fuel cell separator hasbeen illustrated as an example. However, it will be apparent that thepresent invention may be applied to other various fuel cell separators.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but, on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims, andequivalents thereof.

The invention claimed is:
 1. A stainless steel for a polymer fuel cellseparator, comprising C: 0.02 wt % or less, N: 0.02 wt % or less, Si:0.4 wt % or less, Mn: less than 0.2 wt %, P: 0.04 wt % or less, S: 0.02wt % or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %, Ni: 0.8 wt % orless, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, at least one elementselected from the group consisting of V: 0.1 to 1.5 wt %, W: 0.1 to 0.41wt %, La: 0.0005 to 1.0 wt %, Zr: 0.0005 to 1.0 wt %, and B: 0.0005 to1.0 wt %, and waste Fe and inevitably contained elements, wherein Mo isnot added to the stainless steel, wherein a passive film formed on asurface of the stainless steel is formed to have a thickness of 2 to 4.5nm, and wherein the contact resistance of the stainless steel is 10mΩcm² or less under an operating environment of 60 to 150° C.
 2. Thestainless steel of claim 1, wherein the Cr/Fe oxide ratio of a passivefilm formed on a surface of the stainless steel is 1.5 or more in aregion of 1.5 nm or less.
 3. The stainless steel of claim 1, wherein theCr(OH)₃/Cr oxide distribution of a passive film formed on a surface ofthe stainless steel has a ratio of 0 to 0.7 in a region of 1 nm.
 4. Thestainless steel of claim 1, wherein the V is 0.39 to 1.5 wt %.
 5. Astainless steel for a polymer fuel cell, comprising C: 0.02 wt % orless, N: 0.02 wt % or less, Si: 0.4 wt % or less, Mn: less than 0.2 wt%, P: 0.04 wt % or less, S: 0.02 wt % or less, Cr: 25.0 to 32.0 wt %,Cu: 0 to 2.0 wt %, Ni: 0.8 wt % or less, Ti: 0.5 wt % or less, Nb: 0.5wt % or less, at least one element selected from the group consisting ofV: 0.1 to 1.5 wt %, W: 0.1 to 0.41 wt %, La: 0.0005 to 1.0 wt %, Zr:0.0005 to 1.0 wt %, and B: 0.0005 to 1.0 wt %, and waste Fe andinevitably contained elements, wherein Mo is not added to the stainlesssteel, wherein a passive film is formed on a surface of the stainlesssteel, wherein the Cr/Fe oxide ratio of the passive film is 1.5 or morein a region of 1.5 nm or less, and wherein the contact resistance of thestainless steel is 10 mΩcm² or less under an operating environment of 60to 150° C.
 6. The stainless steel of claim 5, wherein the passive filmis formed to have a thickness of 2 to 4.5 nm.
 7. The stainless steel ofclaim 5, wherein the Cr(OH)₃/Cr oxide distribution of the passive filmhas a ratio of 0 to 0.7 in a region of 1 nm.
 8. The stainless steel ofclaim 5, wherein the V is 0.39 to 1.5 wt %.
 9. A stainless steel for apolymer fuel cell separator, comprising C: 0.02 wt % or less, N: 0.02 wt% or less, Si: 0.4 wt % or less, Mn: less than 0.2 wt %, P: 0.04 wt % orless, S: 0.02 wt % or less, Cr: 25.0 to 32.0 wt %, Cu: 0 to 2.0 wt %,Ni: 0.8 wt % or less, Ti: 0.5 wt % or less, Nb: 0.5 wt % or less, atleast one element selected from the group consisting of V: 0.1 to 1.5 wt%, W: 0.1 to 0.41 wt %, La: 0.0005 to 1.0 wt %, Zr: 0.0005 to 1.0 wt %,and B: 0.0005 to 1.0 wt %, and waste Fe and inevitably containedelements, wherein Mo is not added to the stainless steel, wherein apassive film is formed on a surface of the stainless steel, wherein theCr(OH)₃/Cr oxide distribution of the passive film has a ratio of 0 to0.7 in a region of 1 nm, and wherein the contact resistance of thestainless steel is 10 mΩcm² or less under an operating environment of 60to 150° C.
 10. The stainless steel of claim 9, wherein the passive filmis formed to have a thickness of 2 to 4.5 nm.
 11. The stainless steel ofclaim 9, wherein the Cr/Fe oxide ratio of the passive film is 1.5 ormore in a region of 1.5 nm or less.
 12. The stainless steel of claim 9,wherein the V is 0.39 to 1.5 wt %.